Update cov to not to be triggered by external developers. (#7855)

Author: granluo

.github/workflows/test_coverage.yml

@@ -297,7 +297,7 @@ jobs:
         with:
           path: /Users/runner/test
       - name: Compare Diff and Post a Report
-        if: github.event_name == 'pull_request' && ${{ env.METRICS_SERVICE_SECRET }}
+        if: github.event.pull_request.merged != true && env.METRICS_SERVICE_SECRET
         env:
           base_commit: ${{ needs.check.outputs.base_commit }}
         run: |
@@ -308,7 +308,7 @@ jobs:
           swift run CoverageReportGenerator --presubmit "firebase/firebase-ios-sdk" --commit "${GITHUB_SHA}" --token $(gcloud auth print-identity-token) --xcresult-dir "/Users/runner/test/codecoverage" --log-link "https://github.com/firebase/firebase-ios-sdk/actions/runs/${GITHUB_RUN_ID}" --pull-request-num ${{github.event.pull_request.number}} --base-commit "$base_commit"
           fi
       - name: Update New Coverage Data
-        if: github.event.pull_request.merged == true && ${{ env.METRICS_SERVICE_SECRET }}
+        if: github.event.pull_request.merged == true && env.METRICS_SERVICE_SECRET
         run: |
           if [ -d "${{steps.download.outputs.download-path}}" ]; then
           cd scripts/code_coverage_report/generate_code_coverage_report